Hi Diksha,

As we know,

Mole is a term that denotes the Quantity or amount present in that matter

According to the defination 

Mole = Given mass / Molar mass

Lets take an example

Find moles for the 12g of carbondioxide?

So how we will do, just see the given weight (In question) it will be our Given mass 

now we have to calculate Molecular mass of CO₂

_{1x atomic mass of carbbon  + 2 x atmoic mass of oxygen = 1 x 12 + 2 x 16 = 44g (Molar mass)}

Moles = Given mass/Molar mass

Moles = 12g/44g

Moles for 12g carbon dioxide will be 0.2727

Hope you got  your answer

Thanks for asking